Subdural electrodes are placed on the brains of patients in a range of different medical procedures. For example, epileptic patients with medically intractable, that is, drug-resistant, seizure disorders are often evaluated for possible surgical therapy. Most surgical candidates are subject to implantation of subdural metallic electrodes upon the brain surface, or depth electrodes with tips beneath the brain surface, for the purpose of seizure localization by intracranial electroencephalography (“iEEG”) and for mapping of eloquent cortex. Planning surgical margins for resection of epileptic zones is a challenging problem due to the uncertainty associated with subdural electrode positions and inaccurate, labor-intensive techniques for electrode localization. Several techniques have been employed to visualize electrode placement in vivo including metal artifact localization, such as those methods described by M. A. Silberbusch, et al., in “Subdural Grid Implantation for Intracranial EEG Recording: CT and MR Appearance,” AJNR Am. J. Neuroradiol., 1998; 19:1089-1093; three-dimensional reconstruction and x-ray derived location projection, such as those methods described by P. A. Winkler, et al., in Usefulness of 3-D Reconstructed Images of the Human Cerebral Cortex for Localization of Subdural Electrodes in Epilepsy Surgery,” Epilepsy Res., 2000; 41:169-178; and automated template MRI transformation and projection, such as those methods described by D. Kovalev, et al., in “Rapid and Fully Automated Visualization of Subdural Electrodes in the Presurgical Evaluation of Epilepsy Patients,” AJNR Am. J. Neuroradiol., 2005; 26:1078-1083.
Additional methods to visualize electrode placement include x-ray co-registration, such as those methods described by K. J. Miller, et al., in “Cortical Electrode Localization from X-Rays and Simple Mapping for Electrocorticographic Research: The ‘Location on Cortex’ (LOC) Package for MATLAB,” J. Neurosci. Methods, 2007; 162:303-308; curvilinear reformation, such as those methods described by A. H. Schulze-Bonhage, et al., in “Visualization of Subdural Strip and Grid Electrodes Using Curvilinear Reformatting of 3D MR Imaging Data Sets,” AJNR Am. J. NeuroradioL, 2002; 23:400-403; and CT/MRI image co-registration, such as those methods described by R. Grzeszczuk, et al., in “Retrospective Fusion of Radiographic and MR Data for Localization of Subdural Electrodes,” J. Comput. Assist. Tomogr., 1992; 16:764-773, by M. Nelles et al., in “Fusion of MRI and CT with Subdural Grid Electrodes,” Zentralbl. Neurochir., 2004; 65:174-179, and by J. X. Tao, et al., in “The Accuracy and Reliability of 3D CT/MRI Co-Registration in Planning Epilepsy Surgery,” Clin. Neurophysiol., 2009; 120:748-753.
Moreover, methods for visualizing electrode placement also include computer aided stereotactic model creation, such as those methods described by K. Morris, et al., in “A Computer-Generated Stereotactic ‘Virtual Subdural Grid’ to Guide Resective Epilepsy Eurgery,” AJNR Am. J. Neuroradiol., 2004; 25:77-83; and digital 2D photography co-registered to 3D reconstructed MRI, such as those methods described by M. Mahvash, et al., in “Coregistration of Digital Photography of the Human Cortex and Cranial Magnetic Resonance Imaging for Visualization of Subdural Electrodes in Epilepsy Surgery,” Neurosurgery, 2007; 61:340-344, discussion 344-345, and by J. Wellmer, et al., “Digital Photography and 3D MRI-Based Multimodal Imaging for Individualized Planning of Resective Neocortical Epilepsy Surgery,” Epilepsia, 2002; 43:1543-1550. It is noted, however, that methods such as those disclosed by Morris, et al., rely on model creation using so-called virtual ray tracing approaches, in which a model of the cerebral surface is produced by effectively shining a light on the surface of the brain as depicted in post-implantation images. The noted limitation of such methods is that susceptibility artifacts resulting from the presence of implanted electrodes are pervasive in MR images, thereby yielding inaccurate cerebral models when ray tracing is implemented.
More recently, intra-operative fluoroscopy has been used for strip electrode placement, such as those methods described by L. Eross, et al., in “Neuronavigation and Fluoroscopy-Assisted Subdural Strip Electrode Positioning: a Simple Method to Increase Intraoperative Accuracy of Strip Localization in Epilepsy Surgery,” J. Neurosurg., 2009; 110:327-331. These methods are useful in the visualization of grid and strip locations, but lack a degree of accuracy and/or resolution in imaging detail due to interpolation and projection limitations. Many of these techniques also rely on expert guidance and manual marking of electrode positions, making them prone to human measurement error.
Some of the more recently published procedures rely on 2D photographic information regarding electrode position. This introduces discrepancies in position through the estimation of complicated three-dimensional brain geometry with two-dimensional pictorial information. Also, only visible electrodes exposed by the craniotomy defect during the implantation of electrodes can be photographed in the operating room. Thus many electrodes cannot be localized by intra-operative photography, especially strip electrodes placed via small burr hole craniotomies. Most importantly, when the craniotomy defect is closed, brain distortion and cortical compression can physically shift electrodes from their initial photographed positions, as described in previous studies by D. L. Hill, et al., in “Measurement of Intraoperative Brain Surface Deformation Under a Craniotomy,” Neurosurgery, 1998; 43:514-526, discussion 527-518. This compression affects the cortex at the critical period of patient monitoring by iEEG, making quantification of changes in the positioning of the brain and precise measurement of electrode positions following their implantation essential.
X-ray image projection corrects for this brain distortion when x-ray image acquisition follows the closure of the craniotomy defect; however, 2D x-ray images still require elaborate registration techniques for 3D visualization on cortical surfaces, such as those described by S. S. Dalai, et al., in “Localization of Neurosurgically Implanted Electrodes Via Photograph-MRI-Radiograph Coregistration,” J. Neurosci. Methods, 2008; 174:106-115, and are not currently realizable without dedicated experts. The projection of x-ray data onto the brain surface requires a vertex point for back projection and manually defined landmarks. This method ultimately lacks 3D information required for visualizing foreshortened electrode strips oriented along the x-ray path rather than perpendicular to the x-ray beam, and lacks information for medially located electrodes.
Projection of electrode positions onto 3D renderings of brain MRI scans acquired prior to the implantation of the electrodes into the patient is also a common practice found in most previously published methods. Consider, for example, the methods described by S. S. Dalai, et al., in “Localization of Neurosurgically Implanted Electrodes Via Photograph-MRI-Radiograph Coregistration,” J. Neurosci. Methods, 2008; 174:106-115, by J. D. Hunter, et al., in Locating Chronically Implanted Subdural Electrodes Using Surface Reconstruction,” Clin. Neurophysiol., 2005; 116:1984-1987, by M. Mahvash, et al., in “Coregistration of Digital Photography of the Human Cortex and Cranial Magnetic Resonance Imaging for Visualization of Subdural Electrodes in Epilepsy Surgery,” Neurosurgery, 2007; 61:340-344, discussion 344-345, by K. J. Miller, et al., in “Cortical Electrode Localization from X-Rays and Simple Mapping for Electrocorticographic Research The ‘Location on Cortex’ (LOC) Package for MATLAB,”J. Neurosci. Methods, 2007; 162:303-308, and by M. Nelles et al., in “Fusion of MRI and CT with Subdural Grid Electrodes,” Zentralbl. Neurochir., 2004; 65:174-179. This is done because MRI images gathered post-implantation are subject to magnetic field susceptibility artifacts caused by the metal electrodes at the surface of the brain. 3D rendering of electrode-affected images has thus been limited, and subsequently brain shift and compression caused by electrode grids and craniotomy defects have never been addressed in imaging models. However, post-implantation MR images are necessary to visualize the cortical displacement and flattening that affect electrode position relative to the brain surface.
It would therefore be desirable to provide a method for the registration of iEEG electrode locations with a subject-specific 3D brain model that accounts for potential post-implantation brain shifts and is not negatively affected by susceptibility artifacts. Despite the numerous and varied technical solutions developed over a wide span of years, as described above in detail, none have yet been able to provide such a method.